Telephone interface devices such as modems and the like are typically used to couple a telephone line to a user device such as a computer. In general, these devices typically include a device interface for connection to the computer and a telephone line interface for connection to the telephone line. The telephone line interface typically includes a circuit of the type commonly referred to as a data access arrangement (DAA), which transmits and receives various telephone line signals and otherwise provides the standard required telephone line interface.
The circuitry in the interface device can include an isolation amplifier circuit which receives signals from the computer and couples them to the telephone line while providing the required isolation between the user device and the telephone line. The amplifier circuit can include first and second amplification stages optically isolated from and coupled to each other. In typical fashion, the first stage of the amplifier drives an optically emitting element such as a light-emitting diode (LED) which is optically coupled to a photosensitive element such as a photodiode in the second amplifier stage. A reflective dome is typically located over the LED and the photodiode to provide the optical coupling.
Typically, the signal coupled from the user device to the first amplifier stage is a bipolar signal, that is, its level swings between positive and negative voltages. To enable the isolation amplifier to accommodate the bipolar signal without clipping it, the interface device typically includes a prebiasing circuit which continuously applies a DC prebias voltage to the amplifier to raise the DC level of the amplifier such that the bipolar input signals can be processed.
Interface devices can also include switching circuitry, commonly referred to as a "hook switch", which closes a connection to the telephone line in an "off-hook" condition such that current is permitted to flow between the public switched telephone network (PSTN) and the user device. In the "on-hook" condition, the hook switch is open to prevent current flow to the PSTN. A conventional hook switch circuit includes a relay controlled by the user device via a control signal which when activated closes the relay to make the connection with the telephone line and which when deactivated opens the relay.
In some prior systems, the hook switch control signal is optically isolated from the hook switch itself. The control signal activates an optical emitting device such as an LED which is optically coupled to a photosensitive device such as a phototransistor or photodiode. When the hook switch signal is activated, the photodiode is switched to an off-hook state to close the hook switch and thereby connect to the telephone line.
U.S. Pat. No. 5,555,293 to Krause describes an exemplary DAA telephone interface device illustrative of a typical prior art interface configuration. The DAA described in the '293 patent includes an isolation amplifier having optically coupled stages used to transfer signals between the telephone line and subscriber equipment. The device also includes a separate hook switch circuit driven by an off-hook control line. The off-hook control is optically coupled to the hook switch circuit which includes a pair of FET devices. The '293 patent is therefore illustrative of prior art systems which utilize individual and independent circuitry to provide line signal isolation/amplification and hook switch control.
Hence, prior art telephone interface devices, e.g., DAAs, can include a large amount of circuitry to implement and control the interface between the user device, e.g., modem, computer, and the telephone network. Such hardware-intensive systems can be quite expensive; therefore, reduction in the number of electronic components in such systems can be important in reducing overall size and cost.